Horizontal cross-flow scrubber for sulfur oxide removal

ABSTRACT

Sulfur dioxide removal efficiency of a horizontal cross-flow gas liquid contactor is increased by decreasing the interfering spray density.

This invention relates to the removal of sulfur dioxide from industrialwaste gas, for example combustion gas from steam power plants, by wetscrubbing the gas in a horizontal, elongate, gas-liquid contractor withan aqueous absorbent.

Horizontal, elongate spray scrubbers devoid of internal packing areeffective gas-liquid contactors for removal of sulfur dioxide from largevolume flows of waste gas. A particularly effective scrubber of thistype utilizes aqueous absorbent sprays directed across the chambersubstantially perpendicular to the horizontal flow of waste gas as morefully described in U.S. Pat. No. 3,948,608 to Alexander Weir, Jr. Acommercial embodiment of this scrubber has a plurality of spray nozzlespositioned at the top of the scrubber as illustrated in FIG. 1. Thenozzles are arranged in stages as illustrated in FIG. 2 and directaqueous absorbent substantially vertically downward across crosssections of the gas flow path along the length of the scrubber.Typically, from four to six stages are used. Individual headers conveyabsorbent to the nozzles of each stage in an amount necessary to satisfythe gas/liquid flow rate ratio (G/L) required for the particularinstallation. This amount may be, for example, 700 liters per second ineach stage discharged through 50 nozzles having 90 degree cone anglesspaced 0.163 meters apart along the individual header.

The sulfur dioxide removal or absorption efficiency of these horizontalscrubbers is a function of many variables as reported in our articleentitled "The Kellogg-Weir Air Quality Control System", ChemicalEngineering Process, pages 64-65, (August 1977) incorporated herein byreference. In summary of relevant aspects of that article, we expressthe following relationships: ##EQU1## where: D=diffusivity of SO₂ in thegas phase

G=gas volume flow rate

K_(g) =overall mass transfer coefficient

L=liquid volume flow rate per stage

N=number of spray stages

P=outlet concentration of SO₂ in the waste gas

P_(o) =inlet concentration of SO₂ in the waste gas

R=gas constant

T=gas temperature

d=(Sauter) mean diameter of the spray droplets

k_(g) =gas phase mass transfer coefficient

k_(l) =liquid phase mass transfer coefficient

l=mean distance traveled by the spray droplets

m=slope of the equilibrium curve characterizing the gas/liquid pair

u=relative velocity between the spray droplets and the gas

v=mean velocity of the spray droplets.

The relative importance of k_(g) and k_(l) varies not only according tothe choice of absorbent, but also varies according to the sulfur dioxideconcentration existing at any point along the waste gas flow path.

For example, the use of a very effective absorbent such as a 5 weightpercent sodium carbonate solution results in little or no liquid phasemass transfer resistance and, as stated in the reference article, m=0.In this circumstance, equation (2) becomes: ##EQU2##

On the other hand, the use of a relatively ineffective absorbent such asa calcium carbonate slurry results in high liquid phase mass transferresistance throughout most or all of the longitudinal waste gas flowpath primarily because of slow dissolution of calcium carbonate in waterand resulting lower absorption efficiency.

Mass transfer characteristics of other absorbents are generally betweenthe above-mentioned extremes. Quite commonly, a particular system willbe liquid phase mass transfer limited proximate the scrubber gas inletand gas phase mass transfer limited proximate the gas outlet because ofthe decreasing sulfur dioxide concentration along the waste gas flowpath.

Referring to equation (6), one might expect that in a given horizontalscrubber, the efficiency of sulfur dioxide removal will beproportionately increased by increasing the liquid rate in each stageand/or by increasing the number of spray stages. Contrary toexpectation, we have found in gas phase mass transfer limited regionsthat increases in the number of spray stages and/or liquid flow rate donot bring about corresponding increases in sulfur dioxide removal.

We have now found that this anomaly is caused by mutual interference ofspray droplets from proximate spray nozzles. These droplets collide andcoalesce at the initial horizontal plane of interference and for somedistance below that plane until a point is reached where substantiallyall of the droplets fall parallel with each other and no significantfurther interference occurs. In the course of travel, droplet meandiameter increases significantly, as much as by a factor of 4, from theinitial droplet diameter prior to interference. The increased dropletsize results in significant reduction in gas-liquid contact area which,in turn, results in decreased scrubbing efficiency according toequations (1) through (6).

This problem could be avoided by the use of sprays which do notinterfere with each other. In view of the large spray volume ratepreviously recited, however, it is quickly apparent that a horizontalscrubber designed without spray interference would be impracticallylarge.

We have additionally found that some spray interference can existwithout significant detrimental effect on the resulting droplet size. Toquantify this phenomenon, we express the extent of spray interference bythe term "interfering spray density" (I.S.D.) calculated as the averageaqueous absorbent flow rate per unit area at any horizontal plane. Amethod for this calculation is recited later in this specification. Theinterfering spray density (I.S.D.) attains a maximum value at a shortdistance below the horizontal plane of initial interference of the spraydroplets. Most importantly, we have found that the detrimental effectsof spray interference may be significantly reduced by maintaining theI.S.D. below a critical value.

According to the invention, a process and apparatus are provided forremoval of sulfur dioxide from waste gas by passing the gas through ahorizontal, elongate, gas-liquid contactor having a substantiallyunrestricted flow path and passing aqueous absorbent substantiallyvertically downward through the waste gas in a plurality of interferingcones of spray droplets wherein the aqueous absorbent in at least alongitudinal portion of the contactor has a maximum I.S.D. less thanabout 100 liters per second per square meter at any horizontal plane.

FIG. 1 is an elevation view of a typical horizontal, cross-flow SO₂scrubber having an elongate gas-liquid contacting zone 1, waste gasinlet 2, cleaned gas outlet 3, liquid discharge 4 and collection 5means, spray nozzles 6, mist eliminator means 7, and means 8 forcirculation of aqueous absorbent.

FIG. 2 is a three dimensional illustration of the prior art spray nozzlearrangement for the gas-liquid contacting zone of the FIG. 1 scrubberand shows the conventional close nozzle array in stages which providesheets of spray droplets across cross-sections of the chamber.

FIG. 3 illustrates the spray patterns at selected horizontal planeswithin the contacting zone resulting from the spray arrangement of FIG.2. FIG. 3A is a continuation of FIG. 3. In the calculations relevant toFIGS. 2, 3, and 3A, the nozzle spacing (S) is 0.163 meters and thedistance between rows of nozzles (S') is equal to the stage headerdistance (L') which is 3.05 meters.

FIG. 4 is a three dimensional illustration of a nozzle arrangement whichis an embodiment of the present invention and shows a widely spacednozzle array in which the spray nozzles are substantially uniformlyhorizontally spaced apart.

FIG. 5 illustrates the spray patterns at selected horizontal planeswithin the contacting zone resulting from the spray arrangement of FIG.4. FIG. 5A is a continuation of FIG. 5. In the calculations relevant toFIGS. 4, 5, and 5A, the nozzle spacing (S) is 0.47 meters and thedistance between rows of nozzles (S') is 1.02 meters.

FIG. 6 is a graphical representation of the relationship between maximumI.S.D. and the Sauter mean diameter of spray droplets subsequent tointerference. Data for FIG. 6 was developed experimentally using 120degree spiral cone nozzles which produced spray droplets having aninitial mean diameter of 1230 microns (refer to drop size at I.S.D.=0).The curve may be adjusted vertically to extrapolate values for larger orsmaller initial drop sizes.

FIG. 7 is a graphical representation of the I.S.D. existing at varioushorizontal planes for the nozzle arrangements of FIG. 2 and FIG. 4. Thevalues shown were calculated by the method described in the Appendix tothis specification. FIG. 7 shows that the I.S.D. throughout the heightof the contacting zone can be maintained at low values if the maximumI.S.D. which occurs just below the plane of initial interference islimited to a low value.

The waste gas treated by the process and apparatus of this invention issulfur dioxide containing gas in large volume, typically from about 50to about 800 actual cubic meters per second, discharged from sourcessuch as steam power plants, smelters, refineries, pulp mills, orchemical operations. Combustion gas from coal fired power plants isparticularly in point. This gas is typically composed of nitrogen,carbon dioxide, oxygen and smaller amounts of other gases includingsulfur dioxide in concentrations of from about 200 to about 6,000 partsper million by volume. The gas to be scrubbed also normally containsparticulate matter such as fly ash which varies in quantity according tothe waste gas source and the extent of upstream removal by, for example,precipitators.

The gas-liquid contacting zone is, as previously mentioned, ahorizontal, elongate contacting chamber or scrubber having a waste gasinlet at one end and a cleaned gas outlet at the other end. Thecontactor may be internally baffled to direct gas flow in a somewhatsinusoidal flow path in which case the gas flow is, to a degree,countercurrent to the downward flow of aqueous absorbent. Preferably,the contact zone has a horizontal gas flow path with no restriction toeither gas or liquid flow such as packing, trays, mesh, baffles, or thelike.

The chamber is preferably substantially rectangular in cross sectionacross the gas flow path with a height of from about 3 to about 9 metersand a ratio of height to width of from about 0.4 to about 3.0. Thechamber will also have liquid collection and discharge means disposed atthe bottom thereof for further processing and recycle of spent or SO₂--laden absorbent. The collection and discharge means may includereaction tanks that are attached to the scrubber as sumps.

The contacting zone includes a plurality of spray nozzles for aqueousabsorbent positioned at the top of the scrubber to direct acorresponding plurality of interfering cones of spray dropletssubstantially vertically downward through the waste gas. The liquid flowrate for various groups of nozzles along the length of the scrubber andthe total aqueous absorbent flow rate within the contacting zone is afunction of the desired SO₂ removal efficiency as well as the othervariables expressed in Equation (2). Within the longitudinal portion ofthe contacting zone that is gas phase mass transfer limited, a volumeflow rate ratio of waste gas to aqueous absorbent of from about 200:1 toabout 5000:1 per meter of length of the contacting zone is suitable forthe range of SO₂ concentrations typically existing in this region. Weprefer that the spray nozzles be substantially uniformly horizontallyspaced apart so that the aqueous absorbent has a substantially uniforminterfering spray density of any horizontal plane within thelongitudinal portion, not only for control and optimization of liquidflow rate but also to avoid gas channeling in the contacting chamber.

One advantage of a horizontal scrubber of the type described is itscapability to treat a large volume rate of gas at relatively lowpressure drop. Despite this advantage, it is desirable to place somelimit on gas velocity to avoid significant entrainment of aqueousabsorbent in the gas stream and minimize forward sweep of the generallyvertically discharged spray. Gas velocity may range from about 3 toabout 10 meters per second. Parameters for gas velocity, scrubberheight, and spray nozzles should be selected to yield an averagerelative velocity between waste gas and aqueous absorbent of from about7 to about 14 meters per second and a residence time of aqueousabsorbent in the contacting zone of from about 0.3 to about 1.5 seconds.

As previously noted, mass transfer characteristics of aqueous absorbentsfor SO₂ removal systems vary considerably. The active components ofthese absorbents are well known and include sodium carbonate, sodiumsulfite, calcium oxide or hydroxide, and calcium carbonate. The calciumreagents form calcium sulfite and, when oxygen is present, calciumsulfate upon reaction with SO₂ absorbed in water as sulfurous acid. Theyare commercially popular because of their low cost, but when used alone,have relatively high liquid phase mass transfer resistance. When calciumreagents are promoted with soluble sulfates such as magnesium sulfate asdisclosed in U.S. Pat. No. 3,883,639, reactivity of the absorbent isconsiderably enhanced and liquid phase mass transfer resistance isdecreased to a value approaching that of sodium carbonate throughout asubstantial longitudinal portion of the contactor. In such systems,consideration of maximum I.S.D. is applicable throughout most, if notall, the length of the gas-liquid contactor.

Spray droplets originate from the nozzles as distinct cones of spraydroplets having an initial Sauter mean diameter of from about 800 toabout 2000 microns. While a variety of individual nozzle spray patternsmay be utilized, we prefer to use nozzles which form substantiallycircular cones of spray having an included angle of from about 80 toabout 120 degrees. Preferred nozzles have relatively uniform spraydensity, small initial drop size, and high flow/low pressure dropcharacteristics. Preferably, each nozzle discharges aqueous absorbent ata rate of from about 9 to 19 liters per second at an initial dropletvelocity of from about 9 to about 21 meters per second. The nozzlepressure should be sufficient to produce droplets within the meandiameter range recited above and will typically be from about 2 to about3.5 kilograms per square centimeter absolute.

As previously recited, increasing the liquid flow rate in the gas phasemass transfer limited portion of a given scrubber configuration does notproportionately increase SO₂ removal efficiency. The required closernozzle spacing and/or nozzle flow rate results in higher spray densitiesand increased interference among the sprays. Referring now to FIG. 6, itmay be seen that increases in I.S.D. at the horizontal plane of maximumI.S.D. results in radically increased spray droplet size. As previouslynoted, this is due to interference and coalescense among the droplets.Since, according to Equations (1) and (6), SO₂ removal efficiency variesinversely with the droplet size, the significance of spray interferenceand necessity for limitation on the I.S.D. may be appreciated.Accordingly, the specific configuration of sprays and resulting maximumI.S.D. is selected to yield spray droplets having a Sauter mean diametersubsequent to interference of from about 1100 to about 4000 microns toobtain maximum practical gas-liquid contact area.

Since droplet size cannot be decreased subsequent to interference, itfollows that interference must be controlled at the horizontal plane ofmaximum I.S.D. This maximum is best found by calculating I.S.D. atvarious levels in the contacting zone proximate the nozzles. A methodfor I.S.D. calculation is provided in the appendix of thisspecification. Since wide cone angle sprays are used in horizontalscrubbers to avoid bypassing waste gas in the upper part of thecontacting zone, the horizontal plane of maximum I.S.D. will normally befound within about one meter vertical distance from the point of spraynozzle discharge. Considering the SO₂ removal systems previouslydiscribed, the maximum interfering spray density within the contactingzone should not exceed about 100 liters per second per square meter.Where the contacting chamber has a substantially rectangular crosssection, a height of from about 3 to about 9 meters and ratio of heightto width of from about 0.4 to about 3.0 and utilizes circular cones ofspray having an original included angle of from about 80 to about 120degrees, the maximum I.S.D. is preferably about 45 liters per second persquare meter. From a technical viewpoint, there is no lower limit on themaximum I.S.D., however, economic considerations place this value atabout 10 liters per second per square meter. Within the above mentionedparameters the preferred minimum nozzle spacing is about 0.4 meters.Maximum nozzle spacing is limited by considerations of gas channelingand economic scrubber design rather than interfering spray density.Within the range of spray cone angles recited, the maximum preferrednozzle spacing is about 0.85 meters.

The longitudinal portion of the contactor in which the maximum I.S.D. islimited is most effectively that portion in which SO₂ removal is gasphase mass transfer limited. This portion will generally exist proximatethe cleaned gas outlet. Correspondingly, there is generally no gas phasemass transfer limitation proximate the waste gas inlet. In this region,it may be desirable to utilize low G/L ratios and high spray densityaccording to the prior art configuration illustrated in FIG. 2.

With respect to the longitudinal portion of the contactor in which themaximum I.S.D. is limited, ie.--generally the gas phase mass transferlimited portion, one may see from FIG. 4 that the prior art concept ofspray stages becomes moot in considering distribution of gas-liquidcontact area within the chamber. While sprays may be staged withaccompanying large gaps in spray coverage along the length of thescrubber, there is no technical or economic incentive to do so whenusing only one type of aqueous absorbent. Nevertheless, the headerswhich feed groups of spray nozzles are preferably staged for conveniencein piping layout and optimization of liquid flow rates along the lengthof the contacting zone.

To illustrate the invention, Table I provides a summary of calculatedcomparative performance of an SO₂ absorber designed both with the priorart spray arrangement of FIG. 2 and with the FIG. 4 spray arrangementwhich is an embodiment of the present invention. These sprayarrangements result in interfering spray densities, calculated by themethod described in the appendix, that are plotted in FIG. 7 as afunction of the vertical distance below the nozzle arrays. The maximumI.S.D. for the prior art nozzle arrangement is 276 liters per second persquare meter which, according to FIG. 6, corresponds to a droplet meandiameter greater than 2.5×10⁻³ meters. On the other hand, the maximumI.S.D. for the nozzle arrangement of FIG. 4 is only 33.1 liters persecond per square meter corresponding to a droplet mean diameter of1.85×10⁻³ meters. In this comparison, a 5 weight percent solution ofsodium carbonate is utilized as aqueous absorbent since this results ingas phase mass transfer limitation throughout most of the length of thecontacting zone.

Upon application of the tabular values to Equation (6), the SO₂ removalper stage is found to be less than 68% for the prior art nozzlearrangement and 83% for the nozzle arrangement of FIG. 4. In terms of acommercial SO₂ removal system utilizing a horizontal scrubber, thisincrease in performance results in a decrease in the number of spraystages required for nearly complete SO₂ removal from over four to three.The reduction in number of spray stages results in lower initial cost ofthe scrubber installation as well as significantly reduced cost forcirculating the lesser amount of aqueous absorbent.

                  TABLE I                                                         ______________________________________                                        Contactor dimensions:                                                         Width         W         8 meters                                              Height        H         5.5 meters                                            Length (overall)                                                                            L         12.2 meters                                           Stage spacing L'        3.05 meters                                           Gas flow rate G         295 meters.sup.3 /sec.                                Gas velocity  --        6.7 meters/sec.                                       number of nozzles                                                                           --        48                                                    per stage                                                                     Flow rate per nozzle                                                                        --        14 liters/sec.                                        Spray cone angle                                                                            --        120°                                           Initial droplet size                                                                        d° 1.23 × 10.sup.-3 meters                         (Sauter mean diam.)                                                           Droplet travel                                                                              l         6 meters                                              Droplet mean velocity                                                                       v         9 meters/sec.                                         Gas/liquid relative                                                                         u         11.2 meters/sec.                                      velocity                                                                      Gas/liquid volume flow                                                                      G/L       439                                                   rate per stage                                                                SO.sub.2 concentration in                                                                   --        3000 ppmv                                             waste gas                                                                     SO.sub.2 diffusivity                                                                        D         1.67 × 10.sup.-5 meters.sup.2 /                                         sec.                                                  ______________________________________                                        Nozzle arrangement    FIG. 2      FIG. 4                                      Nozzle spacing                                                                              S       0.163 meters                                                                              0.47 meters                                 Nozzle row spacing                                                                          S'      3.05 meters 1.02 meters                                 Maximum Interfering                                                                         Max.    276 lit/sec 33 lit/sec/                                 Spray Density I.S.D.  /m.sup.2    m.sup.2                                     Effective droplet size                                                                      d       >2.5 × 10.sup.-3 m                                                                  1.85 × 10.sup.-3 m                    (Sauter mean diam.)                                                           SO.sup.2 removal exponent                                                                   φ   <1.12       1.77                                        SO.sub.2 removal per stage                                                                          <68%        83%                                         Number of stages re-                                                                        N       >4          3                                           required for 99.5%                                                            SO.sub.2 removal                                                              ______________________________________                                    

APPENDIX Calculation of Interfering Spray Density (I.S.D.)

The following exemplary method for calculation of interfering spraydensity (I.S.D.) is provided with particular reference to FIGS. 3-3A,and 5-5A.

Referring to FIG. 3, part (a) which illustrates a vertical cross-sectionthrough a row of 120° spray cones with S spacing along the row, theinterfering spray density (I.S.D.) at any horizontal plane y is theweighted average of spray densities existing at that plane excluding thefraction having no interference.

The I.S.D. may be calculated as the summation of the multiple,fractional densities that exist at the plane in question: ##EQU3##

SD_(yi) is the spray density for areas of interference involving i conesof spray at the plane y and is equal to iQ/πr_(y) ² where:

i=number of interfering cones of spray

Q=flow rate per nozzle (liters/sec.)

r_(y) =radius of spray cone at plane y (meters)

f_(yi) is the fraction of spray involved in a given interference with icones of spray (i=2,3,4- - - ) at the plane in question and isdetermined from the pattern of interfering sprays at that plane.Referring to FIG. 3, part (c), the fraction of spray that is interferingmay be derived from: ##EQU4## where P=S/2r_(y)

For the prior art spray arrangement of FIG. 2 and the resulting spraypatterns shown in FIGS. 3 and 3A, the Table I values required forcalculation of I.S.D. are:

S=0.163 meters

Q=14 liters/sec.

Spray cone angle=120°

y=variable

r_(y) =y Tan 60°

P=(S/2r_(y))

FIG. 3, (b):

when 0≦r_(y) ≦S/2, there is no spray interference and I.S.D._(y) =0.

FIG. 3, (c) (d): ##EQU5## FIG. 3A, (e) (f):

when S<r_(y) ≦3/2 S, there are both two and three spray interferences.##EQU6##

FIG. 3A, (g):

when 3/2 S<r_(y) ≦2S, there are two, three and four spray interferences.##EQU7##

Results from the foregoing calculations at the horizontal planes y shownon FIGS. 3 and 3A have been plotted in FIG. 7 where it may be seen thatthe spray nozzle arrangement of FIG. 2 has a maximum I.S.D. of 276liters/sec/meter² occurring at a plane 0.08 meters below the nozzles.

For the nozzle arrangement of FIG. 4 and the resulting spray patternsshown in FIGS. 5 and 5A, the Table I values required for calculation ofI.S.D. are:

S=0.47 meters

S'=1.02 meters

Q=14 liters/sec.

Spray cone angle=120°

y=variable

r_(y) =y Tan 60°

P=S/2r_(y)

P'=S'/2r_(y)

FIG. 5, (b):

when 0≦r_(y) ≦S/2, there is no spray interference and I.S.D._(y) =0

FIG. 5, (c) (d):

when S/2<r_(y) ≦S, there is two spray interference. ##EQU8##

FIG. 5A, (e):

when S<r_(y) ≦S'/2, there are both two and three spray interferenceswithin the same row but not interference between adjacent rows. ##EQU9##

FIG. 5A, (f):

when S'/2<r_(y) ≦√S² +S'² /2, there is both two and three sprayinterferences within the same row. Additionally, there is the sprayinterference between adjacent rows. ##EQU10##

Results from the last group of calculations at the horizontal planes yshown on FIGS. 5 and 5A have also been plotted in FIG. 7 where it may beseen that the spray arrangement of FIG. 4 has a maximum I.S.D. of 33liters/sec/meter² occurring through the planes at about 0.2 to about 0.3meters below the nozzles.

We claim:
 1. A process for removing sulfur dioxide from industrial wastegas comprising:(a) passing the waste gas substantially horizontallythrough a horizontal, elongate, gas-liquid contacting zone having asubstantially unrestricted flow path, a waste gas inlet at one end, anda cleaned gas outlet at the opposite end; and (b) passing aqueousabsorbent through the waste gas within at least a longitudinal portionof the gas-liquid contacting zone in a plurality of interfering cones ofspray droplets having an initial Sauter mean diameter of from about 800to about 2000 microns directed substantially vertically downward whereinthe aqueous absorbent has a maximum interfering spray density at anyhorizontal plane within the longitudinal portion less than about 100liters per second per square meter.
 2. A process according to claim 1wherein the interfering cones of spray droplets are formed fromdistinct, substantially circular cones of spray droplets having anincluded angle of from about 80 to about 120 degrees and the spraydroplets have a Sauter mean diameter of from about 1100 microns to about4000 microns subsequent to interference.
 3. A process according to claim2 wherein the spray droplets of the distinct cones have an initialvelocity of from about 9 to about 21 meters per second and the aqueousabsorbent has a residence time in the contacting zone of from about 0.3to about 1.5 seconds.
 4. A process according to claim 2 wherein theaqueous absorbent has a substantially uniform interfering spray densityat any horizontal plane within the longitudinal portion of thegas-liquid contacting zone.
 5. A process according to claim 1 whereinthe waste gas flow rate is from about 50 to about 800 actual cubicmeters per second and the aqueous absorbent contains an active compoundselected from the group consisting of sodium carbonate, sodium sulfite,and a calcium compound which forms calcium sulfite upon reaction withsulfurous acid.
 6. A process according to claim 5 wherein the averagerelative velocity between the waste gas and the aqueous absorbent isfrom about 7 to about 14 meters per second and the volumetric flow rateratio of waste gas to aqueous absorbent within the longitudinal portionis from about 200:1 to about 5000:1 per meter of length of thecontacting zone.